The Environment Agency has confirmed the source of an oil spill in Wallingford last month was from a boat.
An investigation was launched in July following the incident near the Boat House, a riverside cafe.
The spill has been described as "worrying" in a place well-used by swimmers, canoeists, paddleboarders and others.
A spokesperson for the Environment Agency said: “Our investigation into the oil pollution on the River Thames at Wallingford found the source was from a boat.
"We are providing advice and guidance to the boat owner to prevent a repeat of this incident.
“During the incident we worked with South Oxfordshire District Council to help protect bathers and river users from the impacts of the pollution.
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"We closed the bathing beach at Wallingford, and we deployed booms to retain the oil around the boat.
"There are no long-term impacts to the River Thames in Wallingford from the incident.
“Anyone with concerns about pollution in any waterway can call our 24-hour incident hotline, 0800 807060.
"Our Swimfo website has the latest conditions at more than 450 designated bathing waters.”
In response to this, Wallingford town councillor Mike Acreman said: "Any pollution of the River Thames is worrying because the river system provides a source of public water supply and supports a wide diversity of wildlife.
"People swim, boat, canoe and paddle-board in the river and walk along its banks.
"So maintaining high water quality is very important to local people and Wallingford Town Council keeps a close eye on this on their behalf.
"Thankfully [the oil spill] was spotted immediately and was quickly acted upon by the Environment Agency.
"The short-term solution to this type of pollution is for boat owners to be more diligent and to ensure oil and fuel are not leaking from their crafts.
"In the long term, the replacement of fossil-fuel powered boats with electric boats will help with this problem and will contribute to reducing climate change."
Mr Acreman is also part of the River Users Network.
He added: "The recent designation of Wallingford Beach as a bathing area is fantastic but requires us to be even more diligent about pollution.
"The site is now monitored and people are informed about the water quality.
"We can then campaign for further action when pollution is recorded.
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"If we all work together, we will hopefully see a continued upgrade in water quality in the River Thames that has been improving over the past few decades.
"This will enhance the quality of life for us and the creatures we share the river with.
"This is an important aim in Wallingford Town Council's recent Biodiversity Action Plan."
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